In January this year, large parts of southern Australia were ablaze with fierce bush fires, while most of the UK was covered in snow. Half a world away from each other, and at one point nearly 40ºC apart, there aren’t too many similarities to be drawn between the two locations. And yet, there is a water weed, Crassula helmsii, that survives happily in both extremes – and in the UK, where it has been introduced, this adaptability is proving extremely problematic.
Crassula helmsii, also known as Australian swamp stonecrop or New Zealand pygmyweed, is a small semi-aquatic plant in the Crassulaceae family. As its common name implies, this low-growing succulent originates from the antipodes, but was introduced to Britain from Tasmania almost 100 years ago. Initially sold by garden and aquatic centres as an oxygenating plant, by the 1950s it had established in the wild, and from there it has spread to numerous ponds, lakes and waterways throughout the UK.
As you might assume from a plant that can grow happily in both Australia and the UK, Crassula helmsii is highly adaptable: it can tolerate a broad range of temperatures, light and nutrient levels, and utilises the Crassulacean Acid Metabolism (CAM), which allows it to absorb additional carbon dioxide from its environment. Its ability to withstand unfavourable conditions means that it can persist throughout the winter, giving it a competitive edge over many native species. Crassula has three growth forms – terrestrial, emergent and submerged (up to 3 metre depth), and so it can colonise almost any still or slow-moving waterbody and the surrounding banks. Furthermore, Crassula can regenerate from a single fragment as small as 1 cm, readily putting out roots and forming dense mats that may deplete oxygen levels in the water. Not only does this create problems with fishing, boating and recreation, but there may also be ecological impacts. There are concerns that Crassula can out-compete native plants and, once established as the dominant vegetation, will not support the biodiversity that native species do.
So why isn’t Crassula helmsii such a problem in Australia? One contributing factor lies in the Enemy Release Hypothesis. This is the concept that if a species arrives in a new, non-native region, without any of its associated enemies (herbivores or pathogens) that usually regulate it, it will increase in distribution and abundance. Indeed, the lack of natural enemies on Crassula in the UK appears to be a distinct advantage.
This is why scientists at CABI are researching the potential for Classical biological control; finding a highly specific co-evolved insect or pathogen from Crassula’s native range, performing rigorous host-range testing against a pre-selected test plant list, and if suitable, introducing it to the invasive range. Currently the team are in Australia, surveying for natural enemies, while in our quarantine facilities tests continue with a number of previously collected potential candidates.
This approach is not a quick or simple one, but it could provide us with a safe, effective and sustainable long-term solution. Currently, any effective alternatives are few and far between, particularly in highly invaded areas: most chemicals are prohibited from use in and around water; mechanical removal is extremely labour-intensive, with large mats being virtually impossible to eradicate completely. Not only are these alternatives for control costly and often ineffective, but they are completely non-specific and make it impossible to avoid destroying the very same native vegetation that were are trying to protect.
Paradoxically, as land managers are struggling to prevent Crassula from dominating valuable waterbodies, this invasive weed is still legally being sold in garden centres, often mislabelled as Tillaea recurva, thus aiding its continued proliferation across the UK countryside. Understandably, this conflict can make our efforts in trying to manage this growing problem seem a little futile. However, the good news is that by April 2014 DEFRA’s new regulation will come into force across England and Wales, which bans the sale of five non-native aquatic plants, including Crassula helmsii. The prospect of this new legislation helping to reduce the incidence of new invasions gives promise to our endeavours in controlling this problematic weed.
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30 September 2019